Molecular self-assembly of electrically conductive polymers

ABSTRACT

A molecular self-assembly process based on the alternating deposition of a p-type doped electrically conductive polycationic polymer and a conjugated or nonconjugated polyanion has been developed. In this process, monolayers of electrically conductive polymers are spontaneously adsorbed onto a substrate from dilute solutions and subsequently built-up into multilayer thin films by alternating deposition with a soluble polyanion. In contrast to a deposition process involving the alternate self-assembly of polycations and polyanions, this process is driven by the electrostatic attractions developed between the p-type doped conducting polymer and the polyanion. The net positive charge of the conducting polymer can be systematically adjusted by simply varying its doping level. Thus, with suitable choice of doping agent, doping level and solvent, it is possible to manipulate a wide variety of conducting polymers into exceptionally uniform multilayer thin films with layer thicknesses ranging from a single monolayer to multiple layers.

The United States government has rights in the invention by virtue ofGrant Nos. 9023867-CTS and 9022933-DMR from the National ScienceFoundation.

BACKGROUND OF THE INVENTION

The present invention is generally in the area of the fabrication ofmultilayer thin films of electrically conducting polymers.

Ultrathin organic films are currently gaining interest in many areassuch as integrated optics, sensors, friction reducing coatings orsurface oriention layers, as described, for example, by Swalen, et al.,Langmuir 3, 932 (1987) and the Special Issue on Organic Thin Films, Adv.Mater. 3 (1991). Most of these functions require the preparation of welldefined films composed of molecules with appropriate properties in aunique geometrical arrangement with respect to each other and to thesubstrate. Molecularly thin layers, particularly those deposited onelayer at a time, offer the possibility to construct multilayerassemblies in which the distance between two molecules can be controlledin the Angstrom range. As a result, the processing and manipulation ofpolymer monolayers has recently emerged as a viable means to createultra thin polymer coatings with well defined molecular organizations.

The surfaces of various materials can be dramatically modified by thedeposition of monolayers of polymers onto the substrate. Such surfacemodification, for example, can be used to promote adhesion andlubrication, prevent corrosion, modify the electrical and opticalproperties of the material or create electroactive monolayers suitablefor various optical and electronic sensors and devices. In many cases,however, the deposition of a single monolayer is not sufficient toachieve the desired changes in surface characteristics and it isnecessary to coat the substrate with multiple layers of the polymer.This presents a significant problem as typical adsorption processes(both chemical and physical) are self-limiting, thereby only allowingthe deposition of a single monolayer with thicknesses in the range of5-30 Å. Thus, one would like to have the ability to deposit multilayerthin films onto substrates.

There are four principal methods for the preparation of ultrathinmultilayered films: solution casting, Langmuir-Blodgett technique,chemisorption, and the method of Decher, et al. Solution casting ofpreformed bilayer aggregates and annealing of spin coated films ofcopolymers yields layered structures, but the alignment of the layersand the positioning of molecules with respect to each other is limited.In the Langmuir-Blodgett (LB) technique, a film is prepared on thesurface of water and then transferred onto solid substrates. Thismethod, however, is inconvenient for automation and large scaleapplication and is generally only applicable to flat substrates. Anothermethod is based on chemisorption but requires exacting conditions andoftentimes multiple chemical reactions.

Recently, Decher, et al., Thin Solid Films 210/211, 831 (1992) and DE4026978 (WO92-073188/10), have demonstrated that it is possible to buildup multilayer thin films of polymers onto charged surfaces via thealternating deposition of polycations and polyanions. The basis for thismultilayer assembly process is the ionic attraction of the permanentlyfixed charges that exist on the polycations (positive charge) andpolyanions (negative charge). In essence, the excess charge of a polyionadsorbed onto a substrate surface is used to attract a polyion of theopposite charge onto the surface. Multilayer thin films are fabricatedby simply alternating the dipping process.

The self-assembly process as described by Decher and coworkers isillustrated in FIG. 1. In this case, a positively charged glasssubstrate 10, created by suitable silane chemistry, is first immersedinto a dilute solution of a polyanion 12 followed by immersion in adilute solution of a polycation 14. As indicated in the figure,repetition of this cycle produces a multilayer thin film comprised ofalternating layers of polycations 14 and polyanions 12. The thicknessand conformation of each polymer layer deposited are determined by thechemistry of the depositing solution. For example, solutions withrelatively high polyion concentrations or high ionic strengths favor theformation of thicker monolayers deposited in the form of random coilswhereas very dilute solutions produce thinner monolayers with polymerchains adopting a more extended chain conformation.

This approach can be used to manipulate a variety of different polyions,including conjugated polyions (conjugated polymers fitted with ionizablesidegroups). These latter materials, frequently referred to asconducting poymers, are of interest due to their unusual electrical andoptical properties which have their origin in the delocalized electronicstates of the polymer's conjugated backbone. Although layer-by-layerdeposition is possible with conjugated polyions, such materials simplydo not exhibit the range of properties found in conjugated polymers thatdo not contain ionizable sidegroups. In short, the addition of ionizablesidegroups to the repeat structure of a conjugated polymer significantlycompromises the level of conductivity achievable with the polymer andlowers its environmental stability. It is therefore much more desirableand useful to be able to fabricate more conventional conjugated polymerssuch as polyaniline and polypyrrole into ultrathin multilayer thinfilms.

Although these nonderivatized conjugated polymers have been identifiedas the source of many potentially useful electrical and opticalproperties, it is extremely difficult to process the electricallyconductive forms of these materials into technologically useful forms.For example, many applications proposed for conducting polymers, such asmicroelectronic devices, chemical and biochemical sensors,electrochromic displays, anti-corrosion coatings and transparentantistatic coatings, require thin films of electrically conductivepolymers with precisely controlled thicknesses and molecularorganizations. Indeed, it is apparent that significant progress could bemade towards the application of these materials if they could beobtained in large area, thin film forms in which both the thickness andmolecular organization of the film were controllable at the molecularlevel.

Some attempts have been made to accomplish this important goal. Forexample, Milliken Corp has disclosed a procedure for coating varioustextile fibers with uniform, electrically conductive films ofpolypyrrole and polyaniline. Specifically, the deposition of anelectrically conductive coating of polypyrrole onto the fibers isaccomplished by placing the fibers into a dilute aqueous solution ofpyrrole that also contains an oxidizing agent such as ferric chlorideand negative counterions suitable for enhancing the conductivity andconductivity stability of the polymer. The counterions are typicallyadded in the form of sulfonic acids such as naphthalene disulfonic acid.A typical coating solution contains about 10 g/l ferric chlorideanhydride, 5 g/l toluenesulfonic acid and 0.2 g of pyrrole monomer.

Although this chemistry is well suited for coating fibers with thinfilms of conducting polymers such as polypyrrole and polyaniline, it isnot possible to use this process to fabricate multilayer thin films withprecisely controlled thicknesses and layer sequences. Since thechemistry used to deposit a conducting polymer coating cannot becontrolled at the molecular level it is extremely difficult toreproducibly deposit ultra-thin coatings in the range of 10-50 Å thick.In short, the Milliken process and related processes for creating thinfilm coatings of conducting polymers simply do not providelayer-by-layer molecular level control over the deposition process northe structure of the film. The fabrication of multilayerheterostructures of conducting polymers is therefore not possible withcurrently known techniques.

It is therefore an object of the present invention to provide a methodfor producing multilayered thin films of conducting polymers having highelectrical conductivies which are environmentally stable.

It is another object of the present invention to provide multilayeredthin films of conducting polymers with high electrical conductivitieswhich are environmentally stable.

It is still another object of the present invention to provide methodsfor solubilizing p-doped conjugated polymers, and the solutions, for usein making multilayered thin films.

SUMMARY OF THE INVENTION

A molecular self-assembly process based on the alternating deposition ofa p-type doped electrically conductive polymer and a conjugated ornonconjugated polyanion has been developed. In this process, monolayersof electrically conductive polymers are spontaneously adsorbed onto asubstrate from dilute solutions and subsequently built-up intomultilayer thin films by alternating deposition with a solublepolyanion. In contrast to a deposition process involving the alternateself-assembly of polycations and polyanions, this process is driven bythe electrostatic attractions developed between the p-type dopedconducting polymer and the polyanion. The net positive charge of theconducting polymer can be systematically adjusted by simply varying itsdoping level. Thus, with suitable choice of doping agent, doping leveland solvent, it is possible to manipulate a wide variety of conductingpolymers into exceptionally uniform multilayer thin films with layerthicknesses ranging from a single monolayer to multiple layers.

As described in the examples, this process has been used to assemblethin films of electrically conductive polypyrrole, polyaniline, andpoly(3-hexyl thiophene). Conductivities as high as 40 S/cm can bereadily obtained on films containing as few as four deposited layers.Since this is a layer-by-layer deposition process, it is also possibleto use this approach to fabricate complex multilayer thin filmscontaining layers of different conducting polymers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a molecular self-assembly process involving thealternate deposition of polyanions and polycations, as described byDecher, et al. in the prior art.

FIG. 2 displays the optical absorption data generated during theself-assembly of alternating layers of doped polypyrrole and sulfonatedpolystyrene. This figure shows that the broad visible absorption bandcharacteristic of highly conductive polypyrrole increases incrementallywith each additional layer deposited in the multilayer film. The insetof this figure shows that this is a linear process indicating that eachdeposited layer of polypyrrole contributes an equal and reproducibleamount of material to the film.

FIG. 3 is a graph showing an overlay of several optical absorptionversus number of deposited layer plots that illustrate the linear natureof the polypyrrole deposition process for three different solutions, PTSpH 2.5 (dark diamonds); no PTS pH 2.5 (open squares); no PTS pH 2.5,aged (dark squares).

FIG. 4 shows examples of polyanions that can be used to fabricate p-typedoped polypyrrole multilayers in a layer-by-layer manner.

FIG. 5 is a graph showing how the amount of polypyrrole (measured asabsorbance units at 550 nm) deposited onto a surface during a single dipin a polypyrrole bath varies as a function of time (minutes) for glassslides with different surface treatments, hydrophilic/untreated (darksquares), hydrophobic glass (dark diamonds), and SPS treated (opensquares).

FIG. 6 is a graph of the amount of polyaniline adsorbed onto the surfaceof a glass slide (estimated from optical absorption data) as a functionof the number of layers deposited from solutions with differentpolyaniline concentrations: 0.01M (+), 0.005M (squares), 0.001M (darkcircles), and 0.0001M (X), where the polyanion was sulfonatedpolystyrene.

FIG. 7 is a graph showing the amount of polyaniline deposited from 0.01M, pH 2.5 solutions onto negatively charged (+), hydrophilic (opensquares) and hydrophobic (dark circles) glass substrates as a functionof the number of alternating layers of polyaniline/sulfonatedpolystyrene deposited.

FIG. 8 is a graph of the dependence of the amount of polyanilinedeposited per layer (mg/m²) on dipping solution pH.

FIG. 9 is a graph of conductivity (S/cm) versus number of layers forsurfaces which are charged (dark circles), hydrophilic (dark triangles),and hydrophobic (dark squares). This graph shows that a conductivity inthe range of 0.5-1 S/cm is reached with as few as 4 PANi/SPS layers.

DETAILED DESCRIPTION OF THE INVENTION

A very versatile means for fabricating multilayer thin films with novelelectrical and optical properties is disclosed which utilizes amolecular-level, layer-by-layer deposition process. This process isespecially useful for the construction of heterostructure thin filmswith complex molecular architectures and thicknesses that arecontrollable at the molecular level. The basic process used to createalternating layer thin films involves dipping a substrate into a dilutesolution of a p-doped conjugated polymer, rinsing the substrate withwater (or other solvent for the conjugated polymer) and then dipping itinto a dilute solution containing a polyanion. This process can berepeated as many times as desired to build multilayer thin films inwhich each bilayer deposited is only about 10-50 Å in thickness or,optionally, between approximately 10 and 60 Å in thickness, depending onparameters such as solution concentration, doping level, pH, and ionicstrength. Results show that a variety of p-type doped conjugatedpolymers and conjugated and nonconjugated polyions can be deposited,thereby making it possible to fabricate a diverse collection of newmultilayer heterostructures based on conducting polymers.

In contrast to the Decher process in which multilayer thin films arebuilt up in a layer-by-layer fashion via the ionic attractions developedbetween negatively and positively charged polyions (sometimes calledpolyelectrolytes), this process is driven by the attractions developedbetween a positively charged p-type doped conducting polymer andnegatively charged polyion. The positive charges of the conductingpolymer are not created by the ionization of permanently fixedsidegroups such as organic acid or base containing sidegroups, but arerather in the form of partially delocalized defect states that existalong the polymer backbone as a result of doping. Since the number ofthese doping induced defect states created along the backbone (polarons,bipolarons, etc.) depend directly on the oxidation state of theconjugated polymer, they can be systematically varied from none (neutralpolymer, no multilayer assembly possible) to about one every three orfour repeat units (highly oxidized polymer). The ability to readilycontrol the linear density of positive charges along the polymerbackbone via chemical doping provides an additional level of controlover the polymer deposition process. This type of charge control issimply not possible with permanently fixed ionic charges and issurprising since it was not previously recognized that partiallydelocalized charge defect states could be utilized to attract negativelycharged polymers in a multilayer deposition process.

Once it is recognized that the backbone delocalized charges created bychemical doping can be used to fabricate alternating layers ofconducting polymers and negatively charged polyions, it becomesnecessary to develop the chemistry needed to produce dilute solutions ofthese materials in their doped forms. It is well recognized thatnonderivatized, doped conducting polymers are insoluble in mostsolvents, particularly water. In fact, the process of doping aconjugated polymer inevitably renders it insoluble in an aqueoussolution. This is in sharp contrast to conjugated polyions which becomevery water soluble when their sidegroups are ionized into their saltforms.

The fact that conducting polymers do not become highly water solubleupon doping demonstrates that the charge transfer complexes formedduring doping are not simple ionic complexes but are rather a verydifferent state of matter with a unique set of physical and chemicalproperties. The polaronic-like charges that are created by the dopingprocess impart properties to the polymer that are quite different fromthose developed when ionic charges are created by the ionization ofsimple organic acids and bases. Further, it is generally well acceptedthat, in contrast to conventional polyions (including conjugatedpolyions), the doped forms of these materials are not very stable inaqueous solutions.

It has now been found that the generation of water soluble, p-type dopedconducting polymers can be accomplished in a number of different ways.In one embodiment, conducting polymer chains are formed in situ in adilute aqueous solution primarily consisting of a monomer and anoxidizing agent. In this case, the conducting polymer is actuallycreated in the solution and subsequently spontaneously adsorbed onto thesubstrate surface as a uniform, ultra-thin film of between approximately10 and 50 Å in thickness.

Thin, electrically conductive coatings of polypyrrole and polyaniline,for example, can be formed on various substrates by simply placing theobject to be coated in an aqueous bath containing dilute (less thanabout 0.1 m/l) quantities of pyrrole (or aniline) monomer and a suitableoxidizing agent such as ferric chloride or ammonium peroxysulfate. Theuse of dilute solutions of the monomer insures that the electricallyconductive polymer formed from the oxidative polymerization of themonomer will be deposited exclusively onto the substrate to be coated asopposed to simply polymerizing in the solution and precipitating out asan insoluble powder.

Highly uniform and dense multilayer thin films can be easily fabricatedby simply dipping the substrate into a dilute aqueous solution of apolyanion, whereby a monolayer of this material is deposited onto thep-type doped conducting polymer. This process of alternately depositinglayers of a p-type doped polymer and a negatively charged polyanion canbe repeated as often as needed to create thin films with preciselycontrolled thickness and structure.

Alternatively, in a second embodiment, preformed conducting polymers areused directly by forming dilute solutions of their doped forms insuitable solvent systems. In this case, it is necessary to control thetype of solvent system used and the level and type of chemical doping ofthe polymer chains. The general procedure involves first dissolving theundoped polymer in a suitable organic solvent and subsequently dilutingthis polymer solution with a solvent that contains a dopant for thepolymer. This produces a solvent system capable of solvating the dopedpolymer chains. In the case of polyaniline, for example, it has beenfound that dilute aqueous solutions can be easily formed by firstdissolving the nonconducting emeraldine-base form of this polymer indimethylacetamide (DMAc) (or n-methyl pyyrolidone) (NMP) andsubsequently diluting this solution with acidic water such that thefinal solution has a 90/10 water to DMAc volume ratio. Since the finalstep of this process also acid dopes the polymer, the level of dopingcan be easily adjusted by controlling the pH level of the final dippingsolution. Solutions with polyaniline concentrations as high as 0.01 m/lare easily prepared with this procedure. The net result is a stable,water based solution (90% water) of doped polyaniline that is quite wellsuited for molecular self-assembly via alternate deposition withpolyanions.

Using these solutions, it has been found that multilayer thin films ofelectrically conductive polyaniline can be easily constructed byalternately dipping a substrate into a dilute polyaniline solution and adilute polyanion solution. Optical microscopy indicates that theresultant multilayer thin films are homogeneous and uniform at themicron scale.

Details concerning the specific types of materials that can be used inthis molecular self-assembly process are provided below.

Conjugated Polymers

Conjugated polymers represent a relatively new class of materials whoseelectrical and optical properties can be controllably varied over anextremely wide range, oftentimes in a completely reversible manner. Thisis typically accomplished either by chemical or electrochemicaloxidation of the π-system of the polymer backbone or, in some cases, bydirect protonation of the polymer backbone. Through this chemical"doping" process, it is possible to systematically vary the electricalconductivity of these materials from the insulating state to theconducting state. The electrically conductive forms of these materialsare best described as p-type doped polymeric charge transfer salts inwhich the conjugated polymer supports positive charges that aredelocalized over relatively short segments of the backbone, for example,over three to four repeating units for a highly oxidized polymer. Chargeneutrality is maintained by a negatively charged counterion, which isusually derived from the doping agent.

The unique positively charged defect states created on the polymerbackbone by the doping process can exist in many different forms,including as polarons (coupled radical cations), bipolarons (coupleddications) and solitons (noninteracting cations). Such charged defectstates are believed to be the primary charge carriers in these materialsand are therefore responsible for their electrically conductive nature.

Conducting polymers can be in the form of conjugated polyions ornonderivatized conjugated polymers. Nonderivatized conjugated polymers,i.e., those that do not include ionizable sidegroups, are significantlymore environmentally stable than their derivatized polyion counterpartsand also have been found to exhibit much higher electricalconductivities. Examples include conducting polymers such aspolyaniline, polypyrrole and the poly(3-alkylthiophenes). These polymersexhibit significantly better environmental stabilities and much higherelectrical conductivities than similar materials modified to containionizable sidegroups such as sulfonated polyaniline and poly(thiopheneacetic acid).

Specific examples of nonderivatized, p-type doped conjugated polymersthat are particularly useful are shown below. Other examples ofnonderivatized conjugated polymers can be found in "Conjugated PolymericMaterials: Opportunities in Electronics, Optoelectronics, and MolecularEnginering", J. L. Bredas and B. Silbey, Eds., Kluwer, Dordrecht, 1991.##STR1##

Conjugated and Nonconjugated Polyanions

Polyanions in the most general sense represent any type of polymer thatis fitted with ionizable groups (typically within the repeat unit) thatare capable of supporting negative charges when ionized (such ascarboxylic acid and sulfonic acid groups). Examples of these negativelycharged polyelectrolytes abound in the literature and are well known tothose skilled in the art. For example, see, "Coulombic interactions inMacromolecular Systems" ACS Symposium Series No.302, A. Eisenberg and F.Bailey eds., 1986. Typical examples of conjugated and nonconjugatedpolyanions include sulfonated polystyrene, sulfonated polyaniline,poly(thiophene acetic acid), polyacrylic acid, and polymethacrylic acid.Examples of some of these conjugated and nonconjugated polyanions areshown below.

    ______________________________________                                        Nonconjugated Polyanions                                                                     Conjugated Polyanions                                          ______________________________________                                         ##STR2##                                                                                     ##STR3##                                                       ##STR4##                                                                                     ##STR5##                                                      ______________________________________                                    

The present invention will be further understood by reference to thefollowing non-limiting examples.

EXAMPLE 1

Prior Art fabrication of multilayered thin film devices.

The Decher process, illustrated in FIG. 1, can be extended to conjugatedpolymers through the use of conjugated polyions (a conjugated polymerfitted with ionizable sidegroups) such as poly(thiophene acetic acid)and sulfonated polyaniline. These materials, however, do not displayhigh levels of electrical conductivity in their doped forms and are notenvironmentally stable due to the presence of the ionizable sidegroupsthat are needed to fabricate the thin films. Thus, thin films with highelectrical conductivities (greater than 0.1 S/cm) and usefulenvironmental stabilities cannot be fabricated with the Decher process.This was demonstrated by the following example.

Poly(thiophene-3-acetic acid) (PTAA) was prepared by the ferric chloridepolymerization of ethyl thiophene-3-acetate (Lancaster Synthesis)followed by acid hydrolysis of the ester group using the method of A. T.Royappa; A. T. Royappa and M. F. Rubner, Langmuir, 8, (1992) 3168.Sulfonated polyaniline (SPAN) was synthesized using the proceduredescribed by Epstein et al. [J. Yue and A. J. Epstein, J. Am. Chem.Soc., 112 (1990) 2800]. Polyallylamine (PAH) (MW=28,000 g/mole) fromAldrich Chemical Co. was used without further purification.

Solutions containing the polyanions were made by dissolving theappropriate polymer in 0.1M sodium hydroxide and subsequently convertingthe salt solution to an acidic solution (pH 4.5) by adding HCl. Thepolycation solutions were made by dissolving the polymer in a pH 4.5 HClsolution. All solutions were rendered acidic to ensure that the aminegroups present on the polycations remained in their protonated form.

Glass slides with hydrophobic, hydrophilic and positively chargedsurfaces were used as substrates for the adsorption process. Thesesubstrates were all cleaned by first placing them in a hot H₂ SO₄ /H₂ O₂(7:3) bath for 1 hour and then in a H₂ O/H₂ O₂ /NH₃ (5:1:1) bath for 30minutes. The substrates were extensively rinsed with Mili-Q™ water aftereach cleaning step. The above procedure produces hydrophilic glassslides. Hydrophobic surfaces were created by gas phase treatment ofthese slides with 1,1,1,3,3,3 hexamethyl disilazane whereas positivelycharged surface were made by treating the hydrophilic slides with(N-2-aminoethyl-3-aminopropyltrimethoxysilane) solutions.

For the hydrophilic and hydrophobic slides, multilayers were fabricatedby first immersing the substrate in the polycation solution and then inthe polyanion solution. The substrates were dipped in each solution for2 minutes and subsequently washed with a pH 4.5 HCl solution for 15seconds and rinsed in a pH 4.5 HCl bath for 2 minutes. After eachdeposition and cleaning step, the samples were air dried. For thepositively charged glass slides, the substrates were first immersed inthe polyanion solution and then in the polycation solution.

In-plane conductivities were measured on multilayer films deposited ontohydrophilic, positively charged and hydrophobic glass slides using thestandard Van der Pauw 4-point method. For the PTAA/polyallylamine basedmultilayer films, conductivies less than 10⁻⁶ S/cm were measured for theas-prepared films. Similar results were obtained when thin films werefabricated from alternating layers of sulfonated polyaniline andpolyallylamine. This particular multilayer system also exhibits anas-prepared conductivy of less than 10⁻⁶ S/cm. In addition, furtherdoping by immersion in a 1.0M HCL solution only resulted in aconductivity of about 10⁻³ S/cm. In this latter case, the conductivitydecreased very quickly in air and was below 10⁻⁶ S/cm within one day.Thus, as is well documented in the literature, conjugated polyions donot display conductivity levels or stabilities comparable to theirnonderivatized counterparts even after doping.

EXAMPLE 2

Fabrication of multilayer films by alternately dipping a substrate intoan in situ polymerized polypyrrole solution and a polyanion solution.

Solutions suitable for depositing controlled molecular layers ofelectrically conductive polypyrrole were prepared by adding pyrrolemonomer to an aqueous ferric chloride solution (FeCl₃) which had been pHadjusted to a desired level using concentrated HCl. In some cases,p-toluene sulfonic acid was also added to the ferric chloride solution.The dipping solution was aged for 15 minutes and filtered with a 1-2 μmfilter prior to use. A pyrrole concentration of 0.02 m/l was used in allof the following examples. To avoid the need to filter any unwantedpolypyrrole precipitation in the bath, dipping solutions were typicallyused for only 2 to 3 hours.

Multilayer films were fabricated by alternately dipping a substrate intothe in situ polymerized polypyrrole solution and a polyanion solution.The substrates were dipped in the polypyrrole solution for 5 minutes andthe polyanion solution for 10 minutes. Between dips, the substrates werevigorously washed with water and dried under a stream of compressed air.An aqueous sulfonated polystyrene (molecular weight 70,000 g/mole)solution of 0.001 m/l and pH=1.0 was used as the polyanion dippingsolution.

Table 1 displays the conductivity and thickness per bilayer deliveredwhen different pyrrole/FeCl₃ dipping solutions are used to depositconducting layers of polypyrrole in alternation with sulfonatedpolystyrene. This table shows that variations in the chemistry of thepyrrole dipping solution can be used to adjust the level of conductivityof the resultant multilayer thin film and the thickness of the bilayers(polypyrrole/sulfonated polystyrene) used to construct the film.

A number of trends are revealed by these data. First, it can be seenthat for a given FeCl₃ concentration, lowering the pH from 2.5 to 1.0 orto pH 0.0 increases the conductivity of the film. Second, it can be seenthat increasing the FeCl₃ concentration from 0.003M to 0.006M alsoincreases the conductivity of the resultant multilayers. Finally, theseresults show that, for a given FeCl₃ concentration, the addition ofsulfonic acids such as PTS (p-toluene sulfonic acid) can be used toenhance the conductivity of the film.

The thickness per bilayer (polypyrrole plus sulfonated polystyrene) canbe adjusted from about 26 Å to about 44 Å by simply changing theconcentrations of the reactants added to the polypyrrole dippingsolution. Inceasing the dipping time in the polypyrrole bath alsoincreases the thickness of each polypyrrole layer deposited in the film.

Although it has been found that the above indicated concentrations andpH levels are particularly well suited for the molecular level,layer-by-layer deposition of polypyrrole with various polyanions, itwill be obvious to one skilled in the art that further variations inthese levels can be used to adjust and control the conductivity andthickness per bilayer generated with this chemistry.

                  TABLE 1                                                         ______________________________________                                        Conductivity and thickness per bilayer                                        delivered when different pyrrole/FeCl.sub.3 dipping                           solutions are used to deposit conducting                                      layers of polypyrrole in alternation with                                     sulfonated polystyrene.                                                       no PTS           no PTS     0.026M PTS                                        0.003M FeCl.sub.3                                                                              0.006M FeCl.sub.3                                                                        0.006M FeCl.sub.3                                 ______________________________________                                        pH = 2.5                                                                             0.003 S/cm    0.49 S/cm  11.8 S/cm                                              26 Å      36 Å   44 Å                                    pH = 1.0                                                                             2.0 S/cm      7.6 S/cm   24.0 S/cm                                              35 Å      36 Å   44 Å                                    pH = 0 1.0 S/cm      11.5 S/cm  25.2 S/cm                                              35 Å      33 Å   33 Å                                    ______________________________________                                         1. All polypyrrole dipping solutions contained 0.02 m/l pyrrole monomer.      2. The reported thicknesses represent the average thickness of a bilayer      of polypyrrole/sulfonated polystyrene in a 10 bilayer film.                   3. The reported conductivities were measured from films with a total of 1     bilayers.                                                                     4. PTS is ptoluene sulfonic acid.                                        

EXAMPLE 3

Fabrication of device with layers of doped polypyrrole and sulfonatedpolystyrene alternately deposited onto a positively charged substrate.

Layers of doped polypyrrole and sulfonated polystyrene were alternatelydeposited onto a positively charged glass slide and the visibleabsorption spectrum of the resultant film was recorded after eachcomplete bilayer was deposited (polypyrrole plus sulfonatedpolystyrene). The polypyrrole dipping solution consisted of 0.02 m/lpyrrole, 0.006 m/l FeCl₃ and was operated at a pH=1.0. The depositionprocess involved first dipping the glass substrate into the sulfonatedpolystyrene bath for 10 minutes (sulfonated polystyrene bath: 0.001 m/l,pH=1.0), rinsing with water, and then dipping the substrate into thepolypyrrole bath for 5 minutes. This process was repeated to build up amultilayer thin film. The surface of the glass slide was renderedpositively charged by treatment withN-2-aminoethyl-3-aminopropyl-trimethoxysilane.

FIG. 2 displays the optical absorption data generated during theself-assembly of these alternating layers of doped polypyrrole andsulfonated polystyrene. This figure shows that the broad visibleabsorption band characteristic of highly conductive polypyrroleincreases incrementally with each additional layer deposited in themultilayer film. The inset of this figure shows that this is a linearprocess indicating that each deposited layer of polypyrrole contributesan equal and reproducible amount of material to the film. Thisdemonstrates that this is a true layer-by-layer deposition process. Theresultant multilayer thin film was found to display good environmentalstability and exhibit a conductivity of 10 S/cm.

EXAMPLE 4

Comparison of the linear nature of the polypyrrole deposition processfor three different solutions.

FIG. 3 shows an overlay of several optical absorption versus number ofdeposited layer plots that illustrate the linear nature of thepolypyrrole deposition process for three different solutions. In allcases, the polypyrrole dipping solution consisted of 0.02 m/l pyrrole,0.006 m/l FeCl₃ and was operated at a pH=2.5. The polyanion solution wasa 0.001 m/l, pH=1.0 sulfonated polystyrene solution. In one case,p-toluene sulfonic acid was added to this stock solution whereas in theother the solution was simply aged for 4 hours prior to use. These plotsshow that all of these solutions deliver a reproducible amount ofpolypyrrole with each dip, with the PTS solution depositing the greateramount of polypyrrole and the aged solution the least. Thus, an "aged"solution delivers less polymer per layer than a "fresh" solution withthe same chemistry. Note, however, that the deposition process remainsremarkably linear even after the solution has been aged for 4 hours.

EXAMPLE 5

Fabrication of multilayer thin films with polypyrrole and a variety ofdifferent polyanions.

Using the same conditions and process described in Example 2, multilayerthin films were fabricated with polypyrrole and a variety of differentpolyanions. Optical absorption experiments similar to those described inExample 2 again demonstrated that the multilayer thin films were formedin a linear, layer-by-layer fashion with each layer contributing anequal and reproducible amount of material to the film.

Specifically, the absorption band associated with the conducting form ofpolypyrrole was in all cases found to grow in a very linear fashion withthe number of layers deposited in the film. Examples of the conjugatedand nonconjugated polyanions that were tested are shown in FIG. 4.

EXAMPLE 6

Fabrication of highly transparent, electrically conductive coatings onplastic substrates.

This example shows that this process can be used to create highlytransparent, electrically conductive coatings on various plasticsincluding molded parts with complex shapes. Such coatings are ideallysuited for applications requiring transparent anti-static coatings.

Using the deposition process described in Example 2 and a polypyrroledipping solution containing 0.02 m/l pyrrole, 0.006 m/l FeCl₃ and 0.026m/l p-toluene sulfonic acid (the pH of solution was pH=1.0), twobilayers of polypyrrole/sulfonated polystyrene were deposited ontopolystyrene culture dishes and poly(ethylene terephthalate) plasticsheets. The substrates to be coated were first dipped into thesulfonated polystyrene bath followed by dipping into the polypyrrolebath. In both cases, the plastic dishes and sheets were coated with auniform, ultrathin, essentially transparent coating consisting twoalternating layers of polypyrrole/sulfonated polystyrene. The surfaceresistance of these various coated plastic substrates was in the rangeof 10 to 15 kΩ. This value was essentially the same one month after theinitial measurements were made. Thus, with as few as two bilayers it ispossible to create uniform, transparent coatings with very lowresistivities. The adhesion of these coatings to the plastic substrateswas found to be excellent as determined by the standard tape peel test.

EXAMPLE 7

Variation in the amount of polypyrrole deposited onto a surface during asingle dip in a bath as a function of time for substrates with differentsurface treatments.

FIG. 5 shows how the amount of polypyrrole deposited onto a surfaceduring a single dip in a polypyrrole bath varies as a function of timefor glass slides with different surface treatments. In this case, theoptical absorption of polypyrrole at 550 nm was used to monitor theamount of conductive polymer deposited onto the surface. Hydrophobicsurfaces were created by a vapor phase treatment with hexamethyldisilazane. Negatively charged surfaces were created by treatment withN-2-aminoethyl-3-aminopropyl-trimethoxysilane, which binds groups withpositive charges to the surface, followed by the deposition of a singlelayer of sulfonated polystyrene (SPS) by immersion in a 0.01M, pH 2.5SPS solution for 20 minutes. This latter treatment produces a surfacewith excess negative charges from the SPS polyanion. The polypyrroledipping solution contained 0.02 m/l pyrrole, 0.006 m/l FeCl₃ and 0.026m/l p-toluene sulfonic acid (solution pH=1.0).

As demonstrated by the figure, the amount of polypyrrole deposited ontoa surface is greatly enhanced if the surface has excess negative chargesavailable for ionic bonding. For the dipping times found to be ideal formolecular-level deposition of polypyrrole (5 minutes) onto a negativelycharged surface, essentially no polypyrrole is deposited ontohydrophobic and hydrophilic surfaces. Even after 10 minute dippingtimes, the amount of polypyrrole deposited onto glass is significantlylarger for a negatively charged surface than for a hydrophilic orhydrophobic surface. This shows that in situ polymerized polypyrrole isquickly adsorbed onto a negatively charged surface. Thus, the controlleddeposition of ultra-thin films of polypyrrole is most readilyaccomplished on surfaces previously coated with a polyanion.

EXAMPLE 8

Selective deposition of monolayers of polypyrrole onto regions of asurface that are negatively charged.

To further check the selectivity of the polypyrrole deposition process,a glass slide half coated with a monolayer of a polyanion and halfcoated with a monolayer of a polycation was dipped for 5 minutes into asolution containing 0.02 m/l pyrrole, 0.006 m/l FeCl₃ and 0.026 m/lp-toluene sulfonic acid (pH=1.0). The slide was prepared by firstcoating the entire slide with a monolayer of sulfonated polystyrene bydipping in a 0.01M, pH 2.5 SPS solution for 20 minutes, followed bydipping only half of the slide into an aqueous solution of 0.001 m/lpoly(allylamine hydrochloride) for 20 minutes. This latter polymer is apolycation and therefore places excess positive charges on the surfacethat its solution contacts. After dipping the half polyanion/halfpolycation slide completely into the polypyrrole bath, it was found thata uniform, conducting monolayer of polypyrrole was only deposited ontothe region of the slide that had excess negative charges due to thepresence of a surface monolayer of SPS. No deposition at all wasobserved on the region of the slide that had a surface monolayer of thepolycation. This shows that it is possible to selectively depositmonolayers of polypyrrole onto regions of a surface that are negativelycharged and selectively block the deposition of polypyrrole onto regionsof a surface that are positively charged. This means that it is possibleto create selectively patterned regions of a surface that are coatedwith a monolayer of conducting polypyrrole.

EXAMPLE 9

Layer-by-layer deposition via an in situ polymerization approach usingpolyaniline.

Solutions suitable for depositing controlled molecular layers ofelectrically conductive polyaniline via an in situ polymerizationprocess were prepared by adding aniline monomer and p-toluene sulfonicacid to an aqueous ammonium peroxysulfate solution. The dipping solutionwas aged for 15 minutes and filtered prior to use. Layer-by-layerdeposition was achieved by using a solution with an anilineconcentration of 0.027 m/l, an ammonium peroxysulfate concentration of0.0021 m/l and a p-toluene sulfonic acid concentration of 0.026 m/l.Sulfonated polystyrene solutions of 0.001 m/l and pH=1.0 were used asthe polyanion dipping solutions.

Multilayer films were fabricated by first dipping the substrate into thein situ polymerized polyaniline solution for 5 minutes followed bydipping in the sulfonated polystyrene solution for 10 minutes. Betweendips, the substrates were vigorously washed with water and dried under astream of compressed air. Visible spectra recorded during the process offabricating a multilayer thin film onto a glass substrate previouslytreated to place negative charges on its surface revealed that each dipin the polyaniline solution delivered a uniform and reproducible amountof doped polyaniline. This shows that layer-by-layer deposition via anin situ polymerization approach is also possible with polyaniline.

EXAMPLE 10

Fabrication of multilayer films by alternately dipping a substrate intoa doped polyaniline solution and a polyanion solution.

This example shows that the layer-by-layer molecular self-assembly ofp-type doped conducting polymers can also be achieved by using dippingsolutions containing preformed conducting polymers. Solutions containingdoped polyaniline, synthesized using the procedures reported by J-C.Chaing and A. G. MacDiarmid, Syn. Metals, 13 (1986) 193, were preparedby first dissolving 0.47 g of the emeraldine-base form of this polymerin 25 mL of DMAc (dimethylacetamide)with vigorous stirring andsubsequent ultrasonic treatments to insure complete dissolution. Afterall of the polymer was dissolved, 3 mL of this solution was slowly addedwith stirring to 26 mL of pH 3.5 acidic water (acidified with methanesulfonic acid (MeSO₃ H) or hydrochloric acid). The pH of the finalpolyaniline dipping solution was then adjusted to a level of 2.5 using 1mL of pH 1 and 0.33 mL of pH 0 acidic solutions respectively. In orderto make dipping solutions with different polymer concentrations and pHlevels, the concentration of the polyaniline/DMAc stock solution and thepH of the MeSO₃ H or HCl solutions were adjusted accordingly. Polyanionsolutions of sulfonated polystyrene (SPS) were made by stirring the SPSin MeSO₃ H or HCl aqueous solutions. In all cases, a 0.01M, pH 2.5 SPSsolution was used for dipping. All solutions were filtered with 2-4 μmfilter paper before use.

Multilayer films were made by alternately dipping a substrate into adoped polyaniline solution and a polyanion solution. The substrates weredipped in each solution for 5-10 minutes and subsequently washed andrinsed with pH 2-4 solutions for 15 seconds respectively. The pH of thewashing and rinsing solutions was adjusted to be the same as the pH ofthe dipping solutions. After each deposition and cleaning step, thesamples were blown dry with a gentle flow of compressed filtered air.

FIG. 6 shows a plot of the amount of polyaniline adsorbed onto thesurface of a glass slide (estimated from optical absorption data) as afunction of the number of layers deposited from solutions with differentpolyaniline concentrations, where the polyanion was sulfonatedpolystyrene. The linear behavior of these various plots clearlyindicates that, regardless of the polyaniline concentration, theassembled layers of polyaniline each contribute, on average, an equalamount of material to a given film. This is the characteristic signatureof a well behaved layer-by-layer deposition process. These data alsoshow that the amount of polyaniline deposited per layer is stronglydependent upon solution concentration. Profilometry and ellipsometrymeasurements show that the thickness contributed by eachpolyaniline/sulfonated polystyrene bilayer varies from 36 Å (0.01Msolutions) to 12.5 Å (0.0001M solutions) when dipping times of 5 minutesare used for each layer, as demonstrated in Table 2. This againdemonstrates that it is possible to control the deposition process bysimple adjustments in the solution chemistry.

After exposing these films to pH=0 HCl or methane sulfonic acidsolutions, electrical conductivities of about 1 S/cm are obtained afteras few as 4 layers of polyaniline (PANi) have been deposited.

                  TABLE 2                                                         ______________________________________                                        Thickness contributed per bilayer in                                          polyaniline/SPB films fabricated with a                                       dipping time of 5 minutes.                                                                     Thickness per PANi/                                          Solution concentration (M)                                                                     SPS bilayer (Å)                                          ______________________________________                                        0.01             36                                                           0.005            28                                                           0.001            20                                                           0.0001           12.5                                                         ______________________________________                                    

EXAMPLE 11

Deposition of polyaniline and a suitable polyanion onto substrates withdifferent surface characteristics.

FIG. 7 shows the amount of polyaniline deposited from 0.01M, pH 2.5solutions onto negatively charged, hydrophilic and hydrophobic glasssubstrates as a function of the number of alternating layers ofpolyaniline/sulfonated polystyrene deposited as described in Example 10.This figure shows that the well defined layer-by-layer deposition ofpolyaniline and a suitable polyanion can be accomplished with substrateswith different surface characteristics. Thus, it is not necessary topretreat the substrate surface for multilayer fabrication.

EXAMPLE 12

Effect of polyaniline layer thickness on solution pH.

The dependence of the amount of polyaniline deposited per layer ondipping solution pH is illustrated in FIG. 8. As indicated in thefigure, the amount of polyaniline adsorbed per layer increases as pHdecreases. At low pH levels, the charge density along the PANi backboneis high; therefore, more material is adsorbed onto the polyanion layer.At high pH levels, the number of charges along the backbone is smallerand less material is adsorbed onto the polyanion layer.

EXAMPLE 13

Effect of charge on the substrate on polyaniline multilayer fabrication.

In order to verify that the multilayer fabrication process involving thedeposition of alternating layers of p-type doped polyaniline and apolyanion is related to the charges created along the polyanilinebackbone by acid doping, two experiments were performed.

The first study involved the use of a non-charged form of polyaniline(PANi) which was made by simply adjusting a PANi dipping solution to alevel of pH 7. In contrast to the previously described solutionscontaining doped polyaniline having a pH of between 2 and 4 which arestable for months, such a solution is stable only for a few hours; theexperiment was therefore run within this time period.

It was found that the construction of multilayer films was simply notpossible. A single monolayer of polyaniline was initially adsorbed onvarious glass substrates, but subsequent multilayer fabrication viaalternation with a polyanion was not achieved since there were nopositive charges along the polyaniline backbone to attract thepolyanion.

A second experiment was conducted to check if the PANi adsorptionprocess would occur on a positively charged surface. In this case, asubstrate was immersed in a doped PANi dipping solution followed byimmersion in a conventional polycation solution such as a polyallylaminehydrochloride solution.

Again it was found that multilayer fabrication was not successful. Inthis case, only the adsorption of one PANi layer was observed. It hasalso been found that doped polyaniline will not adsorb onto a surfacethat is already positively charged. These experiments prove that themultilayer deposition process requires the alternate deposition of anacid doped form of polyaniline and a polyanion.

EXAMPLE 14

Electrical conductivity of polyaniline/sulfonated polystyrene multilayerfilms as a function of the number of deposited PANi layers deposited oncharged, hydrophilic and hydrophobic substrates.

The electrical conductivity of polyaniline/sulfonated polystyrenemultilayer films doped with 1M HCl was measured as a function of thenumber of deposited PANi layers from multilayer films deposited oncharged, hydrophilic and hydrophobic substrates.

FIG. 9 shows that a conductivity in the range of 0.5-1 S/cm is reachedwith as few as 4 PANi/SPS layers. It also shows that the deposition of asingle layer of polyaniline is not sufficient to render a surface highlyconductive, that is, multilayers are required to create the highestlevel of conductivity.

EXAMPLE 15

Fabrication of highly transparent, electrically conductive coatings onplastics substrates via the self-assembly of p-type doped polyanilineand a polyanion.

This example shows that highly transparent, electrically conductivecoatings on various plastics, including molded parts with complexshapes, can be created via the self-assembly of p-type doped polyanilineand a polyanion. Such coatings are ideally suited for applicationsrequiring transparent anti-static coatings.

Using the deposition process described in Example 10 and a polyanilinedipping solution containing 0.001 m/l polyaniline (pH 2.5, adjusted withmethane sulfonic acid), three bilayers of polyaniline/sulfonatedpolystyrene were deposited onto polystyrene culture dishes andpoly(ethylene terephthalate) plastic sheets. The substrates to be coatedwere first dipped into the sulfonated polystyrene bath (0.001 m/l, pH2.5 (adjusted with methane sulfonic acid)) followed by dipping into thepolyaniline bath (10 minute dips). Between dips, the substrates wererinsed with pH 3.5 methane sulfonic acid solutions.

In all cases, the plastic dishes and sheets were coated with a uniform,ultrathin, essentially transparent, light green coating comprised ofthree alternating bilayers of polyaniline/sulfonated polystyrene. Thesurface resistance of these various coated plastic substrates was in therange of 2-5M Ω after additional doping by immersion in a pH 0 methanesulfonic acid solution and remained essentially the same one month afterthe initial measurements were made.

Thus, with as few as three bilayers, it is possible to create uniform,transparent coatings with low resistivities. The adhesion of thesecoatings to the plastic substrates was found to be excellent.

EXAMPLE 16

Deposition of electrically conductive coatings of polyaniline onto awide variety of different substrates via a layer-by-layer depositionprocess.

This example shows that electrically conductive coatings of polyanilinecan be deposited onto a wide variety of different substrates via alayer-by-layer deposition process. Using the deposition proceduredescribed in Example 10 and a polyaniline dipping solution containing0.01 m/l polyaniline (pH 2.5, adjusted with methane sulfonic acid), fiveto ten alternating bilayers of polyaniline/sulfonated polystyrene andpolyaniline/sulfonated polyaniline were successfully deposited onto thefollowing substrates: platinum coated glass slides, gold coated glassslides, silver coated glass slides, aluminum coated glass slides, indiumtin oxide coated glass, mica, graphite, tygon tubing (both inside andoutside surfaces) and plexiglass. In all cases, uniform thin coatingswere obtained.

EXAMPLE 17

Stability of the electrical resistance of 10 bilayer thin films ofpolyaniline/sulfonated polystyrene that were self-assembled onto glasssubstrates.

The electrical resistance of 10 bilayer thin films ofpolyaniline/sulfonated polystyrene that were self-assembled onto glasssubstrates using the procedure outlined in Example 10 was tested forstability at elevated temperatures. In this case, samples were furtherdoped after fabrication with either pH 0 HCL or methane sulfonic acid(MSA) solutions. The results are shown in Table 3 below.

                  TABLE 3                                                         ______________________________________                                        Stability of Electrical Resistance of                                         Polyaniline/sulfonated polystyrene thin films                                 at various times and temperatures.                                            Temp./                                                                        Exposure Time                                                                            MSA Doped Sample                                                                             HCl Doped Sample                                    ______________________________________                                        25° C.                                                                            2.6 MΩ   4.8 MΩ                                        40° C./1 hr                                                                       3.8 MΩ   15 MΩ                                         40° C./3 hr                                                                       1.3 MΩ   11 MΩ                                         90° C./1 hr                                                                       1.0 MΩ   87 MΩ                                         90° C./3 hr                                                                       1.5 MΩ   180 MΩ                                        ______________________________________                                    

These results show that HCl doped samples are not very stable andquickly lose their conductive nature at elevated temperatures. MSA dopedsamples, on the other hand, are very stable. In fact, treatment at 40°C. for 3 hours or 90° C. for 3 hour actually slightly increases theirconductivity.

EXAMPLE 18

Multilayer fabrication with a preformed p-type doped conducting polymerdissolved in a nonaqueous solution.

This example shows that multilayer fabrication can be accomplished witha preformed p-type doped conducting polymer dissolved in a nonaqueoussolution. In this case, a conducting polymer dipping solution was madeby adding 1.0 ml of a 0.017 m/l solution of FeCl₃ in nitromethane to a5×10⁻⁴ m/l poly(3-hexylthiophene) in chloroform solution. This producesa deep blue nitromethane/chloroform dipping solution which containsdissolved chains of p-type doped poly(3-hexylthiophene). Multilayerswere successfully fabricated by first dipping a glass substrate,previously treated to carry a negatively charged surface, into thepoly(3-hexylthiophene) solution for 5 minutes and then into an aqueouspolyanion solution containing 0.001 m/l of sulfonated polystyrene at pH=1.0 for 5 minutes. After the polyanion dip, the substrate was rinsedwith water, dried and immersed in a 0.017 m/l FeCl₃ /nitromethanesolution for 10 minutes to redope the conducting polymer which dedopesin the aqueous polyanion bath. This process was repeated to createmultilayer thin films in which each bilayer contains an equal amount ofdeposited poly(3-hexylthiophene).

EXAMPLE 19

Fabrication of heterostructure thin films.

To demonstrate that it is possible to fabricate multilayer thin filmswith controllable layer sequences, heterostructure thin films werefabricated using the procedures described in the above examples. Thesecomplex multilayer films were fabricated by simply dipping a substrateinto a solution containing the specific polymer to be deposited followedby the deposition of a polymer of the opposite charge. As long aspositively and negatively charged polymers are alternately depositedonto the substrate surface, heterostructures of any type and form can beeasily fabricated.

The following layer sequences were deposited to demonstrate thisprocess. The successful deposition of each conjugated polymer layer wasconfirmed by visible spectroscopy.

1) 5 bilayers of poly(thiophene acetic acid)/poly(allylamine)alternating with 5 bilayers of polypyrrole/sulfonated polystyrene to atotal of 30 layers.

2) 5 bilayers of sulfonated polystyrene/poly(allylamine) alternatingwith 5 bilayers of polyaniline/sulfonated polystyrene to a total of 30layers.

3) 2 bilayers of polyaniline/sulfonated polystyrene alternating with 2bilayers polypyrrole/sulfonated polystyrene to a total of 10 layers.

4) alternating bilayers of polyaniline/sulfonatedpolystyrene--polypyrrole/sulfonated polystyrene--poly(thiophene aceticacid)/poly(allylamine) and sulfonated polyaniline/poly(allylamine) to atotal of 30 layers.

In summary, the ability to readily fabricate multilayer thin films ofp-type doped conjugated polymers via a molecular self-assembly processopens up completely new vistas with regard to the thin film processingof conducting polymers and related electroactive materials. Through thealternate deposition of doped conjugated polymers andconjugated/nonconjugated polyanions it is now possible to fabricate anunprecedented number of complex thin film architectures with completecontrol over the supramolecular organizations of the resultantheterostructures. Such films can be used to fabricate and examine newthin film devices and sensors and to tailor the electrical and opticalproperties of various surfaces at the molecular level.

Modifications and variations of the method and devices of the presentinvention will be obvious to those skilled in the art from the foregoingdetailed description. Such modifications and variations are intended tocome within the scope of the following claims.

We claim:
 1. A method for construction of a thin film heterostructurecomprisingdipping a substrate into a solution comprising less than about0.1 m/l of a p-doped conjugated polymer, rinsing the substrate with asolvent for the p-doped conjugated polymer to form a monolayer ofconducting polymer on the substrate, and dipping the coated substrateinto a solution containing a polyanion to form a monolayer of thepolyanion bound electrostatically to the conducting polymer monolayer,wherein a bilayer is formed of the polyanion and p-doped conjugatedpolymer monolayers which is between approximately 10 and 60 Å inthickness.
 2. The method of claim 1 wherein the dipping steps arerepeated using the same or different polymers to form a multilayerheterostructure formed of multiple bilayers, each bilayer having athickness of between 10 and 60 Å.
 3. The method of claim 1 wherein thesubstrate is selected from the group consisting of fibers, plates, andfilms.
 4. The method of claim 1 wherein the polyanion is a conjugatedpolymer.
 5. The method of claim 1 wherein, prior to dipping thesubstrate in the solution comprising the p-doped conjugated polymer, aportion of the substrate is coated with a material having a chargepreventing subsequent deposition of p-doped conjugated polymer monolayeronto the portion of the substrate.
 6. The method of claim 1 wherein thesubstrate which is dipped into the p-doped conjugated polymer is coatedwith a polyanion prior to being dipped.